1. Field of the Invention
The present invention relates to orthopaedic implants and, more particularly, to a method of making orthopaedic implants having a porous surface connected thereto by a process utilizing an organic binder compound.
2. Description of the Related Art
Orthopaedic implants of known design may be constructed, e.g., of cobalt-chromium-molybdenum or titanium alloys. Such materials provide suitable physical characteristics of strength, corrosion resistance, wear properties and biocompatability for use in orthopaedic applications.
It is also known to provide an orthopaedic implant with a porous surface at the exterior thereof. The porous surface may be used to promote bone ingrowth and thereby enhance implant fixation within the bone. Alternatively, the porous surface may receive bone cement therein to enhance implant fixation within the bone. Such porous surfaces may be constructed, e.g., of metal beads or metal fibers which are sintered, diffusion bonded, or welded to the implant to form an integral part of the implant.
Presently, fiber metal mesh used to form a porous surface is pressed into a desired shape and maintained under pressure during the sintering process in which some of the fibers are bonded together to form a pad. The process may also be referred to as diffusion bonding. The metal pad is shaped to correspond to its supporting surface and is then positioned in contact with an implant and clamped in place during a sintering process. Alternatively, the fiber metal pad may be gravity sintered, thereby eliminating the use of external clamping forces. A similar process may be employed when making a porous surface using metal beads.
Sintering the porous surface layer to the implant with external pressure is time consuming and expensive for the manufacturer. During sintering, the ramp up and cool down time for the furnace is approximately 14 hours per cycle. If the porous surface layer is being connected, for example, to the interior bone engaging surface of a femoral knee component, it may take 4 complete cycles. The complex geometric interior design of the femoral knee component requires that only one or two pads be attached during one cycle. The typical interior of the femoral knee defines 5 distinct surfaces which require a porous coating. Therefore, to completely bond all of the porous surface layers to the interior of the femoral knee component would require in excess of 56 hours of furnace time. Added to that time is the time required to connect the clamp tooling to the implant for holding the pad in contact with the implant. From the above description, it is clear that providing a porous surface layer on an implant using existing technologies is time consuming and expensive for the manufacturer of orthopaedic implants.
The present invention provides a method of making an orthopaedic implant having a porous surface by utilizing a water-soluble protein compound such as gelatin to enhance bonding of the porous surface to the implant. Preferably, the gelatin includes an alloying element that is diffused into the metallic particles and lowers the melting temperature of the metallic particles at the interface surfaces by raising the carbon content at the surface of the metal particles. Alternatively, the porous surface layer could be fiber metal mesh impregnated with or otherwise coated by the gelatin. If the porous surface is formed from the plurality of metal wires or fiber metal mesh as it is commonly known, the process includes forming a pad of fiber metal and then impregnating the pad with the gelatin binder. The impregnated pad is then placed in contact with an implant and then gravity sintered.
Regardless of whether the porous layer is formed from a plurality of beads or a layer of fiber metal mesh wire during presintering and sintering, the binder exhibits specific temperature dependent phases. Initially, after the binder is coated over the porous surface layer, or after the impregnated porous layer is applied to the implant, the implant, porous layer, and binder are allowed to dry. Drying causes the binder to become very hard and forms an initial temporary bond between the porous layer and the implant. As the furnace ramps up in temperature, the binder forms a carbon frame-work with the thin porous layer and implant. As the temperature of the furnace continues to increase, some of the carbon becomes defused into the surface of the wires making up the fiber metal mesh. The increased carbon content of the wires decreases the melt temperature of the wires at their surface and causes the wires to fuse or melt bond at contact points with other wires or the implant. Further, if the wires are not in direct contact, the carbon frame-work formed by the binder may assist the melting metal to bridge. Eventually, all of the carbon is defused into the wire and the volatile constituents in the binder are removed leaving the resultant implant substantially free from binder debris. By using the binder and method of the current invention, all of the porous surfaces may be connected to the implant at the same time. As the binder dries and hardens, the binder alone is sufficient to hold the porous surface layers in contact with the implant. Therefore, only one furnace cycle is required to bond a plurality of porous surface layers to the implant. Further, since the binder lowers the melting point of the surface of the wires making up the fiber metal mesh, sintering can be completely accomplished in a shorter sintering cycle and at a lower temperature. Finally, since the binder forms melt bridges between adjacent and the contacting fibers, the bonding within the porous layer is more complete.
In another version of the invention, a plurality of metallic particles are mixed with a water-soluble protein mixture and are spread over the surface of an implant to form a beaded porous surface layer for the implant. For instant, the beads and binder may be poured into a mold to form an outer porous shell of an acetabular cup. The shell is attached to a body of an orthopaedic implant as by sintering or the shell may be sintered separately and placed within an injection mold device to form the outer porous surface of an injection molded polyethylene cup.
In yet another variation of the invention, the binder is used to secure a layer of fine beads to the surface of a fiber metal pad. The fine layer of beads provides a greater contact surface for later sintering the pad to the implant using the binder. This variation could be accomplished by spreading a layer of small beads along the implant surface and then overlying the layer of beads with a layer of fiber mesh. The fiber mesh and beads could then be coated or impregnated with the binder material and then processed according to the teachings set forth above. Alternatively, the bead/fiber metal combination could be presintered together utilizing the binder method of the above invention and then sintered as a unit to the implant, again using the teachings of the subject application. The value of the combination of fiber metal and small beads as described resides in the increased surface area to contact and bond with the implant yet provides the porous fiber metal mat for contact with bone or cement.
While it is believed that the binder alone will be adequate to hold the porous surface layer against the implant, there may be instances or areas on the implant when it may be advantageous to spot weld the pad to the implant to provide initial fixation prior to sintering.
In all variations of the invention, it is important the binder be formed from a protein compound such as gelatin. Gelatin is especially attractive as a binder agent due to its ease of use in a manufacturing environment. The gelatin binder is easy to apply as it does not require any special handling equipment, and it is non-toxic and otherwise safe to handle. Furthermore, if the gelatin is applied incorrectly, it can be washed off with warm water without any damage to the implant or porous surface.
The invention comprises, in another form thereof, a method of forming an orthopaedic implant having a porous surface layer. An orthopaedic implant has a surface configured to support the porous surface layer. A mask includes a cut-out configured to receive a portion of the implant therein. The implant is placed within the cut-out and thereby masked. A water soluble binder solution is sprayed onto at least a part of the unmasked portion of the implant using a sprayer, thereby forming a binder layer on the implant. A porous layer including a plurality of metallic particles is contacted with the binder layer. The porous layer is bonded with the surface of the implant with a sintering process.
The invention comprises, in yet another form thereof, a mask for covering a portion of an orthopaedic implant during manufacture. At least two blocks of material have adjoining surfaces. At least one of the blocks has a cut-out in the corresponding adjoining surface. The cut-outs are configured to receive the portion of the implant therein. At least one fastening device fastens the at least two blocks together.
The invention comprises, in still another form thereof, a binder solution for application to an orthopaedic implant to bind a metallic porous layer with at least a portion of the implant. The binder solution is a mixture including gelatin and water. Preferably, glycerine is added as a plasticizer. Also preferably, alcohol is added to facilitate a uniform mist to be applied to form a uniform coating of binder. More preferably, a colorant is added to facilitate visually gauging binder coating thickness.
An advantage of the present invention is that external forces (and associated machinery) are not required to hold the porous layer to the implant during the sintering operation.
Another advantage of the present invention is that external forces (and associated machinery) are not required to hold the shell defining the porous surface during the sintering operation.
Another advantage is that the shell can be moved from one location to another prior to the sintering operation without damaging the physical integrity thereof.
Yet another advantage is that the binder may include an alloying material which is diffused into the metallic particles, thereby lowering the melting point at the interface surfaces of the metallic particles which is less than the melting point of the material from which the metallic particles are initially constructed.